Abstract By noninvasively measuring temperature distributions at depth, we could provide new vision of thermal processes occurring in tissues well below the skin surface. An instrument with such ability would respond to many clinical needs, including the monitoring of brain status during cardiac surgery, detect abnormal metabolism in the early formation of aggressive breast cancer, and observe the inflammatory processes in arthritis. Microwave radiometry has been used for many years in astronomy to observe temperature of stars far away. Recent advances in microwave electronics have produced ultra-low noise amplifiers, multilayer compact antennas, and ultra-precise analog-to-digital converters which provide powerful new capabilities for radiometric detection and for the miniaturization of such devices. The proposed instrument development combines these elements to develop a novel array of miniature microwave sensors that will reliably and noninvasively detect small fluctuations in heat emission from deep below the surface of skin. A multidisciplinary team will implement an array of compact, ultrasensitive microwave radiometers mounted directly on deep penetrating antennas. With real-time correlation of radiometric signals from multiple antennas and frequency bands we will reconstruct the thermal distributions at depth several cm below the sensors. We will then demonstrate its potential use in medicine, by comparing in ten patients the head thermal profiles obtained noninvasively by a miniature radiometric array prototype with invasive temperature sensors currently used for long-term monitoring of brain status with traumatic injuries. The proposed thermal imager will enable noninvasive detection of thermal phenomena, such as tumors metabolism in breast tissue or inflammatory response deep below the skin with capabilities that extend far beyond current technology and give us an innovative tool for medical applications.